Refactor and move prepare_input_tensors (#1712)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1712

This new version:
- Can fail non-fatally
- Uses RAII to clean up the memory
- Uses separate files for lean and aten modes
- Has tests
- Lives somewhere other than util.h

Reviewed By: larryliu0820

Differential Revision: D52645309

fbshipit-source-id: b117269046efb137982b214b6877b13376b721db